Which is officially only supposed to be used by businesses, i.e. by people who have registered for a business account on the Deutsche Post website:
Für die Nutzung von Warenpost International XS, S, M oder L zum Einzeltarif benötigen Sie lediglich eine Registrierung als Geschäftskunde im Shop der Deutschen Post. Dies können Sie dort im Login-Bereich vornehmen. Mit der Registrierung und Nutzung von Warenpost International zum Einzeltarif verpflichten Sie sich automatisch, eine Mindestmenge von 5 Sendungen pro Quartal zu versenden. Die Frankierung erfolgt im Shop ausschließlich über die Internetmarke.
To use Warenpost International XS, S, M or L at the single rate, you only need to register as a business customer in the Deutsche Post shop. You can do this there in the login area. By registering and using Warenpost International at the single rate, you automatically agree to send a minimum of 5 items per quarter. Franking is done in the shop exclusively via the Internet stamp.
So the option "Warenpost International" is only shown in business accounts, if you registered as a "Privatkunde" you don't even get to see it.
The good news is that they don't check whether you're really a business, nor do they keep track whether you really send 5 packages per quarter.
So there is nothing to stop somebody who isn't a business to register a business account with Deutsche Post (change to tab "Geschäftskunde"): https://shop.deutschepost.de/shop/registration/registrationCollectData.jsp
Once you have registered and have logged into your new "business account" here: https://shop.deutschepost.de/shop/login_page.jsp
you will see the "Warenversand" --> "Warenpost International".